1. Field of the Invention
The present invention relates to a zoom lens for a variable magnification copying machine. More particularly, the present invention relates to a zoom lens for suitably performing a zooming operation while a conjugate length (the distance between object images) from an object face to a formed image face is constantly held.
2. Description of the Related Art
It is possible to very efficiently treat documents for business by a copying machine, especially, an electrophotographic copying machine using plain paper. In the old main current of the copying machine, there was an equal magnification copying machine in which an original image and a hard copy image are equal to each other in size ratio so as to provide equal magnification. However, recently, there have been various kinds of needs with respect to the copying machine. A strong requirement with respect to the copying machine is to provide a function for obtaining a hard copy in a state in which the above size ratio is changed, i.e., a variable magnification function for obtaining copies enlarged or reduced in size.
In this case, there are various kinds of systems with respect to the variable magnification function. As an example, there is a system in which the magnification is changed by using a lens for equal magnification. As a concrete means of this example, there is a structure in which the magnification is changed by changing a conjugate length from a lens having a fixed focal distance to an object face and a conjugate length from this lens to a formed image face, i.e., the distance between object images. There is also another structure in which a converter lens is inserted on an optical path of the lens having the fixed focal distance when the image is enlarged or reduced in size.
When the magnification is changed by changing the above conjugate lengths, it is necessary to move a reflection mirror, etc., so as to change the conjugate lengths. These conjugate lengths are minimal at the time of the equal magnification and are increased as the magnification of the image is different from the equal magnification. Accordingly, when the magnification is changed in a wide range, the moving amounts of the reflection mirror, etc. are very large so that an entire optical system is large-sized. Further, there is a problem that the number of constructional parts is increased and it is necessary to dispose many members manufactured with especially high accuracy.
When the magnification is changed by inserting the converter lens on the above optical path, only one magnification is changed by one converter lens. Accordingly, the magnification is changed by stages by preparing a plurality of converter lenses for enlargement and reduction magnifications having high frequency in use to enlarge e.g., size B5 to A4 or reduce size A3 to A4 with respect to the image. Therefore, when the enlargement and reduction sizes are specified, for example, requirements for unifying and keeping the copy sizes can be satisfied, but the needs of users cannot be completely satisfied.
Further, these converter lenses are constructed by at least two lenses so that cost is high and these lenses must be prepared in accordance with a plurality of magnification stages. Accordingly, a mechanism for switching these lenses becomes complicated so that the cost of the entire copying machine is increased.
Therefore, a zoom lens for performing the zooming operation while the conjugate length from the object face to the formed image face is constantly held has been developed. Japanese Patent Application Laying Open (KOKAI) No. 61-151604 shows one example of such a zoom lens.
In this optical system, a group of lenses equal to those composed of concave, convex and concave lenses are symmetrically arranged around a diaphragm respectively on object and image face sides and are constructed such that first to sixth lenses are sequentially arranged from the object side. The first and sixth lenses construct two fixed lens groups. The second and third lenses construct a first movable lens group. The fourth and fifth lenses construct a second movable lens group. The above optical system is thus constructed by these four lens groups.
At the time of the equal magnification, the first to sixth lenses are located such that these lenses are respectively arranged symmetrically around the diaphragm on the object and image face sides. At the time of variable magnification, the first and second movable lens groups are moved by an equal amount around the diaphragm to enlarge and reduce the image in size.
However, in the variable magnification optical system shown in Japanese Patent Application Laying Open (KOKAI) No. 61-151604, F/No. is about 11 and therefore shows a dark state. Further, the magnification is changed by moving symmetrically the movable lens groups around the diaphragm. Therefore, the moving amount of each of the lens groups is small when the magnification is changed in the vicinity of the equal magnification, but is increased as the magnification of the image is different from the equal magnification on the enlargement or reduction side. In particular, this moving amount is rapidly increased approximately when the enlargement magnification approaches 1.414 times or the reduction magnification approaches 0.707 times. Therefore, it is necessary to secure large distances between the respective lens groups so that the entire lens length becomes very long. Further, at the time of the variable magnification, various kinds of performances for forming images especially at a magnification different from the equal magnification are greatly deteriorated.
The magnification is changed by symmetrically moving the movable lens groups by few lens groups such as four group lenses at the time of the variable magnification. Therefore, there are only four distances between lens faces for changing the focal distances of these lenses. Further, the number of parameters usable for correcting aberration is small so that the aberration cannot be sufficiently corrected.